When you receive a "515 error” that looks similar to this when trying to print an Invoice, or the program just crashes and closes, then check Options > Revision History, from the Order tab. Look at the Reason for Revision column and check if the text has quotations. If it does, then double click on that line and remove the quotation(s) from the text.

The Invoice should then be able to print. Please mention to the customer that they should avoid putting quotations in the reason field when creating the revision to avoid this issue.